Multi-termination scheme interface

ABSTRACT

In an embodiment, a method includes programming a control signal that specifies a target resistance and a target voltage in a circuit. The method further includes sending the control signal to at least one transistor configured to control a current flow in the circuit. The method further includes providing, as an output, a signal with the target voltage and target resistance.

BACKGROUND

Modern digital systems rely on the phase-locked loop (PLL) to provide high speed clocks to synchronize the circuit operations. Most widely used PLL architectures need to use a high quality lower frequency reference clock. A reference clock receiver, therefore, is an essential block of these modern digital systems to provide the high quality reference clock.

SUMMARY

In an embodiment, a method includes programming a control signal that specifies a target resistance and a target voltage in a circuit. The method further includes sending the control signal to at least one transistor configured to control a current flow in the circuit. The method further includes providing, as an output, a signal with the target voltage and target resistance.

In an embodiment, the at least one transistor includes a first transistor, a second transistor, and a third transistor. The first transistor is coupled to allow current to flow between a voltage source and a first resistor. The second transistor is coupled to allow current to flow between ground and a second resistor. The third transistor is coupled to allow current to flow between ground and a third resistor. The first, second, and third resistors are further coupled with a printed circuit board (PCB) trace, the PCB trace receiving a transmission from a driver. The product of a first resistance of the first resistor and a second resistance of the second resistor divided by the sum of the first resistance and the second resistance produces the target resistance. The second resistance divided by the sum of the first resistance and the second resistance times a voltage of the voltage source is also the target voltage.

In an embodiment, a first control signal, configured to be in a first state, coupled to a gate of the first transistor is configured to allow current to flow through the first transistor. A second control signal, configured to be in a second state, coupled to a gate of the second transistor is configured to allow current to flow through the second transistor. A third control signal, configured to be in a third state, coupled to a gate of the third transistor is configured to prevent current from flowing through the third transistor.

In an embodiment, a first control signal, configured to be in a first state, coupled to a gate of the first transistor is configured to prevent current from flowing through the first transistor. A second control signal, configured to be in a second state, coupled to a gate of the second transistor is configured to allow current to flow through the second transistor. A third control signal, configured to be in a third state, coupled to a gate of the third transistor is configured to allow current to flow through the third transistor.

In an embodiment, a first control signal, configured to be in a first state, coupled to a gate of the first transistor is configured to prevent current from flowing through the first transistor. A second control signal, configured to be in a second state, coupled to a gate of the second transistor is configured to prevent current from flowing through the second transistor. A third control signal, configured to be in a third state, coupled to a gate of the third transistor is configured to prevent current from flowing through the third transistor.

In an embodiment, the method further includes generating a first gate signal, a second gate signal, and a third gate signal to input to each respective transistor based on a received first control signal and a second control signal. The first gate signal is generated based on a OR operation of the second control signal and the inverse of the first control signal. The second gate signal is based on the second control signal. The third gate signal is based on a AND operation of the inverse of the first control input and the inverse of the second control input.

In an embodiment, a system includes at least one transistor in a circuit configured to receive a control signal that specifies a target resistance and a target voltage in the circuit. The circuit provides, as an output, a signal with the target voltage and target resistance.

In an embodiment, a system for providing a power down circuit includes a first, second, third, fourth, and fifth transistors. The first transistor is coupled to allow current flow between a voltage source and a first resistor. The second transistor is coupled to allow current flow between a second resistor and a ground. The third transistor is coupled to allow current flow between the voltage source and an inverter. The fourth transistor is coupled to allow current flow between the ground and the inverter. The fifth transistor is coupled to allow current flow between a voltage source and an output of the inverter.

In an embodiment, the second transistor and third transistor are configured to allow current when receiving a power down signal, and the first transistor, fourth transistor, and fifth transistor are configured to allow current in response to an inverted signal of the power down signal.

In an embodiment, a method of providing a power down circuit includes providing a power down circuit having a first, second, third, fourth, and fifth transistors. The first transistor is coupled to allow current flow between a voltage source and a first resistor. The second transistor is coupled to allow current flow between a second resistor and a ground. The third transistor is coupled to allow current flow between the voltage source and an inverter. The fourth transistor is coupled to allow current flow between the ground and the inverter. The fifth transistor is coupled to allow current flow between a voltage source and an output of the inverter.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments.

FIG. 1 is a block diagram illustrating an example embodiment of a pad (PAD) coupled with an on-chip termination (OCT) module and an electrostatic discharge (ESD) module.

FIG. 2 is a block diagram illustrating an example embodiment of on-chip termination employed by the present invention.

FIG. 3 is a diagram illustrating an example embodiment of a receiver configured for AC-coupled low-voltage positive-referenced emitter coupled logic (LVPECL) with on-chip termination.

FIG. 4 is a diagram illustrating an example embodiment of a receiver configured for DC-coupling high speed current steering logic (HCSL) with on-chip termination.

FIG. 5 is a diagram illustrating an example embodiment of a receiver configured for external termination.

FIG. 6 is a diagram illustrating an embodiment of a power down circuit that can be employed in an embodiment of the present invention.

DETAILED DESCRIPTION

A description of example embodiments follows.

One challenge of designing a reference clock receiver is accommodating different termination schemes with different common mode and voltage swing level requirements. In an embodiment of the present invention, a reference clock receiver can be configured for low-voltage positive-referenced emitter coupled logic (LVPECL) and high speed current steering logic (HCSL) with on-chip termination. A person of ordinary skill in the art can recognize that the reference clock receiver can further be configured to accommodate other logic configurations for on-chip termination as well as off-chip termination configurations, such as Complementary Metal Oxide Semiconductor (CMOS), Current-Mode Logic (CML), Low-Voltage Differential Signaling (LVDS), or Transistor to Transistor Logic (TTL). In another embodiment, the reference clock receiver can be configured as a general reference clock receiver with external termination to accommodate all the I/O standards, including LVPECL and HCSL.

FIG. 1 is a block diagram 100 illustrating an example embodiment of a pad (PAD) (e.g., a physical metal pad) coupled with an on-chip termination (OCT) module and an electrostatic discharge (ESD) module (e.g., a protection block). The blocks shown can be implemented in a single-ended embodiment 150, or in a differential embodiment (i.e., the entire FIG. 1). In embodiments, the OCT module includes novel and nonobvious improvements to reference clock receivers.

The ESD of the single ended embodiment 150 is coupled with a capacitor 102, which is coupled to an inverter 106 in parallel with a resistor 104. The output of the inverter 106 is coupled with inverters 108, 110, and 112.

Similarly, the second ESD module of the differential embodiment is coupled with a capacitor 122. The capacitor 122 is coupled with an inverter 126 and resistor 122 in parallel. The output of the inverter 126 is coupled with inverters 128, 130, and 132.

Further, the differential embodiment is connected at the outputs of respective inverters 110 and 130 and inputs of respective inverters 112 and 132. In particular, the output of inverter 110 and input of inverter 112 are both coupled with the input of an inverter 114 and output of an inverter 116. The output of inverter 130 and input of inverter 130 are coupled with the output of inverter 114 and input of inverter 116.

FIG. 2 is a block diagram 200 illustrating an example embodiment of on-chip termination 250 employed by the present invention. The OCT module 250 receives input from the PAD (e.g., from a driver through the PAD). A Transistor (T₁) 202 is coupled to allow current flow from a voltage source to a resistor R₁ 204 in response to the appropriate gate signal “lvpecl_enb”. A second transistor (T₂) 210 is coupled to allow current flow from a resistor R₂ 206 to a ground in response to the appropriate gate signal “oct.” A third transistor (T₃) is coupled to allow current flow from a resistor R₃ to a ground in response to the appropriate gate signal “hcsl_en”. The three resistors, R₁ 204, R₂ 206, and R₃ 208 are coupled, at the ends that are not coupled with their respective transistors T₁, T₂, and T₃, to each other. In this embodiment, the three resistors, R₁ 204, R₂ 206, and R₃ 208 further have a same resistance, which is 100 ohms.

With this setup, the three signals to the gates of the transistors (e.g., “lvpecl_enb,” “oct,” and “hcsl_en”) can determine the mode that the OCT module 250 allows. This is due to a logic circuit that provides the appropriate signals based on an input signal indicating whether on chip termination is enabled (e.g., the “oct” signal), and a signal indicating whether HCSL should be enabled (e.g., the “hcsl” signal). The logic circuit includes an inverter 220, AND gate 222, and OR gate 222. A person of ordinary skill in the art can determine the operation of the logic circuit shown. Table 1 below indicates the preferred outcomes based on the OCT and HCSL digital signals.

TABLE 1 Signal Outcomes OCT = 0 OCT = 1 HCSL = 0 External Provide On-Chip Termination Termination with LVPECL configuration HCSL = 1 External Provide On-Chip Termination Termination with HCSL configuration

Further, Table 2 below indicates the signal values output by the logic circuit to accomplish these outcomes.

TABLE 2 Gate Signals OCT = 0 OCT = 1 HCSL = 0 hcsl_en = 0 hcsl_en = 0 lvpecl_enb = 1 lvpecl_enb = 0 HCSL = 1 hcsl_en = 0 hcsl_en = 1 lvpecl_enb = 1 lvpecl_enb = 1

A person of ordinary skill in the art can extrapolate that different transistors may require inverted values to achieve the desired results in Table 1, for example, by replacing PMOS transistors with NMOS transistors or vice versa.

When “oct” is set to 1, however, the “hcsl” signal governs which mode is used by the OCT module 250.

When “hcsl” is 0, “hcsl_en” and “lvpecl_enb” are both set to 0. This causes current to flow through transistors T₁ 202 and T₂ 210, as therefore also resistors R₁ 204 and R₂ 206. The effect of this circuit configuration is further illustrated in FIG. 3.

When “hcsl” is 1, “hcsl_en” and “lvpecl_enb” are both set to 1. This causes current to flow through transistors T₂ 210 and T₃ 212, and therefore also resistors R₂ and R₃ 208. The effect of this circuit configuration is further illustrated in FIG. 4.

However, when the “oct” signal is 0, all of the transistors are turned off, and no current flows through them. The OCT module 250 effectively acts as a short between PAD and ESD. In this setting, the designer expects that external termination accommodates the termination standard being used. The effect of this circuit configuration is further illustrated in further detail in FIG. 5.

FIG. 3 is a diagram 300 illustrating an example embodiment of the receiver configured for AC-coupled LVPECL with on-chip termination. In this embodiment, the “oct” signal of FIG. 2 is set to 1, and the “hcsl” signal is set to “0.” This activates the LVPECL mode of the OCT module 250 of FIG. 2.

Therefore, the board 301 of FIG. 3 illustrates the LVPECL driver connected to the chip 303 via capacitors 306 and 308 and circuit elements introducing impedance of 50 ohms. Resistors 302 and 304 further couple each of the LVPECL lines to ground. The operation of the board 301 itself is unaffected by the “oct” and “hcsl” signals, however, the chip 303 is configured to receive a board 301 configured for such an LVPECL driver.

Chip 303 having the OCT module behaves as descried in relation to FIG. 2. FIG. 3 shows a simplified version of its operation, with the transistors that allow current to flow being simplified to short circuits, and transistors that are closed to allow current to flow being open circuits. Therefore, each line is connected between two resistors 312 and 314, or 310 and 316, respectively. Resistors 312 and 310 are connected to a voltage source and resistors 314 and 316 are connected to ground. A person of ordinary skill in the art can recognize that FIG. 3 illustrates the differential embodiment, having two OCT modules receiving two separate signals.

FIG. 4 is a diagram 400 illustrating an example embodiment of the receiver configured for DC-coupling HCSL with on-chip termination. The board 401, in this example, provides an HCSL driver. The driver provides two differential lines, each having respective resistors 402 and 404 and impedance circuit elements providing impedance of 50 ohms. In this embodiment, the “oct” signal of FIG. 2 is set to 1, and the “hcsl” signal is set to 1. This activates the HCSL mode of the OCT module of FIG. 2.

Therefore, the board 301 of FIG. 4 illustrates the HCSL driver connected to the chip 403 via resistors 402 and 404 and impedance elements. Chip 403 having the OCT module behaves as described in relation to FIG. 2. FIG. 4 illustrates a simplified version of its operation, with the transistors that allow current to flow being simplified to short circuits and transistors that are closed to allow current to flow being open circuits. Therefore, as described in relation to FIG. 2, the transistors allowing current (T₂ and T₃) allow current through respective resistors R₂ and R₃, which are both connected to ground. Effectively, this simplifies to both R₂ and R₃ being in parallel, which is the same as one 50 ohm resistor. This simplification is snow in FIG. 4 in 50 ohm resistors 406 and 408 connected to each of the differential lines.

FIG. 5 is a diagram 500 illustrating an example embodiment of the receiver configured for external termination. An advantage of on-chip termination is reducing the connector and package reflection. As a result, the receiver can have a better input clock signal. Further, a lower cost results for the receiver by having fewer internal (and therefore more external) parts. Therefore, drivers may be designed for external termination for this reason, and the OCT module 250 of FIG. 2 should accommodate those drivers that have external termination as well. Therefore, an external termination mode is desirable in certain scenarios and the OCT Module 250 should be able to accommodate such a mode when provided by the driver shown in FIG. 2.

As shown by FIG. 2, when “oct” is 0, regardless of the “hcsl” being 0 or 1, all of the transistors do not allow current to flow to either the voltage source or ground. Therefore, with respect to FIG. 5, board 501 illustrates a general driver having two differential lines. Each line has respective impedance elements having an impedance of 50 ohms, respective capacitors 502 and 504, and respective resistor pairs 506 and 508 and 512 and 514 coupled to ground, one of each pair coupled to the differential line from either a voltage source or ground.

On the chip 503 side, each differential line is effectively connected to the ESD module via a short due to all of the resistors shown in FIG. 2 being turned off to current flow. In effect, this allows for external termination to occur off the chip 503, for example, within the general driver or on the board 501.

FIG. 6 is a diagram 600 illustrating an embodiment of a power down circuit that can be employed in an embodiment of the present invention. Diagram 600 illustrates two differential lines having two PAD, OCT, and ESD modules coupled to respective capacitors. For the first differential line, the capacitor is coupled to a tri-state inverter 622, which is coupled to a series of inverters 624, 626, and 628, where the outputs of each respective inverter are connected to the inputs of the next inverter in series, respectively. A transistor T_(1A) 602 is coupled to allow current to flow from a voltage source to a resistor R_(1A) 601, which is coupled to the input of the tri-state inverter 622, as well as the output of the capacitor on the first differential line. A transistor T_(2A) 604 is coupled to allow current to flow from ground to a resistor R_(2A) 603, which is coupled to the input of the tri-state inverter 622, as well as the output of the capacitor on the first differential line and the input to the inverter 622. Transistor T_(1A) 602 receives a power down bar (“pdb”) signal at its gate, while Transistor T_(2A) receives a power down (“pd”) signal at its gate.

The tri-state inverter 622 further is coupled to Transistor T_(3A) 606 and Transistor T_(4A) 608. Transistor T_(3A) 606 is coupled to allow current from the voltage source to the tri-state inverter 622, and Transistor T_(4A) 608 is coupled to allow current to flow from ground to the tri-state inverter 622. Transistor T_(3A) 606 is coupled to receive signal “pd” at its gate, and Transistor T_(4A) 608 is coupled receive to signal “pdb” at its gate. When current flows from Transistor T_(3A) 606 and Transistor T_(4A) 608 through the tri-state inverter 622, the tri-state inverter is in a low impedance state and allows current to flow from its input (e.g., the capacitor) to its output (e.g., to transistor 624). However, when current does not flow, it is in a high-impedance state and does not allow current to flow from its input to its output. The input of the tri-state inverters are biased at mid-rail to prevent the input MOSFET gate from breaking down during power-down with the presence of a large-amplitude clock.

Inverter 624 is coupled to receive the output of the tri-state inverter 622. The inverter 622 is further coupled to a Transistor T_(5A). Transistor T_(5A) is coupled to allow current to flow from the voltage source to inverter 624. The output of inverter 624 is coupled to the input of inverter 626, which itself is coupled to the input of inverter 628.

Inverter 624 is coupled to receive the output of the tri-state inverter 622. The inverter 622 is further coupled to a Transistor T_(5A). Transistor T_(5A) is coupled to allow current to flow from the voltage source to inverter 624. The output of inverter 624 is coupled to the input of inverter 626, which itself is coupled to the input of inverter 628.

The circuit, therefore, has two states based on the two signals to the respective transistors. A person of ordinary skill in the art can recognize that power down and power down bar are opposite signals (i.e., power down bar=0 when power down=1, and power down bar=1 when power down=0). Analyzing the first differential line, when power down is set to 1, current flows through Transistor T_(1A) 602, Transistor T_(2A) 604, and Transistor T_(5A) 610. Current does not flow through Transistor T_(3A) 606 and Transistor T_(4A) 608, which puts the tri-state inverter 622 in a high-impedance state. The output of the tri-state inverter 622 then is pulled to the voltage source/power supply through transistor T_(5A) 510.

Alternatively, when power down is set to 0 and power down accordingly set to 1, current does not flow through Transistor T_(1A) 602, Transistor T_(2A) 604, and Transistor T_(5A) 610. Current does flow through Transistor T_(3A) 606 and Transistor T_(4A) 608, which puts the tri-state inverter in a low-impedance state and allows current to flow from its input to its output, through in-series inverters 624, 626, and 628.

The second differential line is configured in a similar manner to the first differential line described above. For the second differential line, the capacitor is coupled to a tri-state inverter 632, which is coupled to a series of inverters 634, 636, and 638, where the outputs of each respective inverter are connected to the inputs of the next inverter in series, respectively. A transistor T_(1B) 612 is coupled to allow current to flow from a voltage source to a resistor R_(1B) 611, which is coupled to the input of the tri-state inverter 632, as well as the output of the capacitor on the second differential line. A transistor T_(2B) 614 is coupled to allow current to flow from ground to a resistor R_(2B) 613, which is coupled to the input of the tri-state inverter 632, as well as the output of the capacitor on the first differential line and the input to the inverter 622. Transistor T_(1B) 612 receives a power down bar (“pdb”) signal at its gate, while Transistor T_(2B) receives a power down (“pd”) signal at its gate.

The tri-state inverter 622 further is coupled to Transistor T_(3B) 616 and Transistor T_(4B) 618. Transistor T_(3B) 616 is coupled to allow current from the voltage source to the tri-state inverter 632, and Transistor T_(4B) 618 is coupled to allow current to flow from ground to the tri-state inverter 632. Transistor T_(3B) 616 is coupled to receive signal “pd” at its gate, and Transistor T_(4B) 618 is coupled to receive signal “pdb” at its gate. When current flows from Transistor T_(3B) 616 and Transistor T_(4B) 618 through the tri-state inverter 632, the tri-state inverter 632 is in a low impedance state and allows current to flow from its input (e.g., the capacitor) to its output (e.g., to transistor 624). However, when current does not flow, the tri-state inverter 632 is in a high-impedance state and does not allow current to flow from its input to its output. The input of the tri-state inverters are biased at mid-rail to prevent the input MOSFET gate from breaking down during power-down with the presence of a large-amplitude clock.

Analyzing the second differential line, when power down is set to 1, current flows through Transistor T_(1B) 612, Transistor T_(2B) 614, and Transistor T_(5B) 620. Current does not flow through Transistor T_(3B) 616 and Transistor T_(4B) 618, which puts the tri-state inverter 632 in a high-impedance state. The output of the tri-state inverter 632 then is pulled to the ground through transistor T_(5B) 620.

Alternatively, when power down is set to 0 and power down accordingly set to 1, current does not flow through Transistor T_(1B) 612, Transistor T_(2B) 614, and Transistor T_(5B) 620. Current does flow through Transistor T_(3B) 616 and Transistor T_(4B) 618, which puts the tri-state inverter in a low-impedance state and allows current to flow from its input to its output, through in-series inverters 634, 636, and 638.

In addition, in a differential embodiment, the output of inverter 626 and input of inverter 628 are both coupled to an input of an inverter 642 and an output of an inverter 644. The output of inverter 636 and input of inverter 638 are coupled to an output of inverter 642 and input of inverter 644.

While example embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments encompassed by the appended claims. 

What is claimed is:
 1. A method comprising: programming a plurality of control signals including a first control signal and a second control signal that specify a target resistance and a target voltage in a circuit; generating a plurality of gate signals including a first gate signal, a second gate signal, and a third gate signal, the first gate signal is generated based on a first logic operation of the first control signal and the second control signal, the second gate signal being based on the second control signal alone, and the third gate signal being based on a second logic operation of the first control signal and the second control signal; sending the plurality of gate signals to (a) a first transistor coupled to allow current to flow from a voltage source and through a first resistor from a first terminal of the first resistor to a second terminal of the first resistor, (b) a second transistor coupled to allow current to flow from a first terminal of a second resistor to a second terminal of the second resistor to a ground, and (c) third transistor coupled to allow current to flow from a first terminal of a third resistor to a second terminal of the third resistor to the ground, wherein the second terminal of the first resistor, the first terminal of the second resistor, and the first terminal of the third resistor are coupled at a common node; and providing, as an output, a signal with the target voltage and target resistance.
 2. The method of claim 1, wherein: the first resistor, the second resistor, and the third resistor are further coupled with a printed circuit board (PCB) trace, the PCB trace receiving a transmission from a driver; a product of a second resistance of the second resistor and a third resistance of the third resistor divided by a sum of the second resistance and the third resistance produces the target resistance; the second resistance divided by the sum of the first resistance and the second resistance times a voltage of the voltage source is the target voltage.
 3. The method of claim 2, wherein: the first control signal, configured to be in a first state, coupled to a gate of the first transistor is configured to allow current to flow through the first transistor; the second control signal, configured to be in a second state, coupled to a gate of the second transistor is configured to allow current to flow through the second transistor; a third control signal, configured to be in a third state, coupled to a gate of the third transistor is configured to prevent current from flowing through the third transistor.
 4. The method of claim 2, wherein: the first control signal, configured to be in a first state, coupled to a gate of the first transistor is configured to prevent current from flowing through the first transistor; the second control signal, configured to be in a second state, coupled to a gate of the second transistor is configured to allow current to flow through the second transistor; a third control signal, configured to be in a third state, coupled to a gate of the third transistor is configured to allow current to flow through the third transistor.
 5. The method of claim 2, wherein: the first control signal, configured to be in a first state, coupled to a gate of the first transistor is configured to prevent current from flowing through the first transistor; the second control signal, configured to be in a second state, coupled to a gate of the second transistor is configured to prevent current from flowing through the second transistor; a third control signal, configured to be in a third state, coupled to a gate of the third transistor is configured to prevent current from flowing through the third transistor.
 6. The method of claim 1, wherein: the first logic operation represents an AND operation of the second control signal and an inverse of the first control signal, and the second logic operation represents an OR operation of the inverse of the first control signal and the inverse of the second control signal.
 7. The method of claim 1, wherein the second gate signal being based on the second control signal alone includes the second gate signal being at least one of: the second control signal, an inversion of the second control signal, and a delay of the second control signal.
 8. A system comprising: a first transistor in a circuit, a second transistor in the circuit, and a third transistor in the circuit, and a signal generation circuit configured to generate a plurality of gate signals, the plurality of gate signals including a first gate signal, a second gate signal, and a third gate signal, based on a plurality of control signals that specify a target resistance and a target voltage in the circuit, the plurality of control signals including a first control signal and second control signal, wherein the signal generation unit is further configured to generate the first gate signal based on a first logic operation of the first control signal and the second control signal, the second gate signal being based on the second control signal alone, and the third gate signal being based on a second logic operation of the first control signal and second control signal; wherein the first transistor is coupled to allow current to flow from a voltage source and through a first resistor from a first terminal of the first resistor to a second terminal of the first resistor, a second transistor is coupled to allow current to flow from a first terminal of a second resistor to a second terminal of the second resistor to a ground, and the third transistor is coupled to allow current to flow from a first terminal of a third resistor to a second terminal of the third resistor to the ground, wherein the second terminal of the first resistor, first terminal of the second resistor, and the first terminal of the third resistor are coupled at a common node, the circuit providing, as an output, a signal with the target voltage and target resistance.
 9. The system of claim 8, wherein: the first resistor, the second resistor, and the third resistor are further coupled with a printed circuit board (PCB) trace, the PCB trace receiving a transmission from a driver; a product of a second resistance of the second resistor and a third resistance of the third resistor divided by a sum of the second resistance and the third resistance produces the target resistance; the second resistance divided by the sum of the first resistance and the second resistance times a voltage of the voltage source is the target voltage.
 10. The system of claim 9, wherein: the first control signal coupled to a gate of the first transistor is configured to allow current to flow through the first transistor; the second control signal coupled to a gate of the second transistor is configured to allow current to flow through the second transistor; a third control signal coupled to a gate of the third transistor is configured to prevent current from flowing through the third transistor.
 11. The system of claim 9, wherein: the first control signal coupled to a gate of the first transistor is configured to prevent current from flowing through the first transistor; the second control signal coupled to a gate of the second transistor is configured to allow current to flow through the second transistor; a third control signal coupled to a gate of the third transistor is configured to allow current to flow through the third transistor.
 12. The system of claim 9, wherein: the first control signal coupled to a gate of the first transistor is configured to prevent current from flowing through the first transistor; the second control signal coupled to a gate of the second transistor is configured to prevent current from flowing through the second transistor; a third control signal coupled to a gate of the third transistor is configured to prevent current from flowing through the third transistor.
 13. The system of claim 9, wherein: the first logic operation represents an AND operation of the second control signal and the inverse of the first control signal, and the second logic operation represents an OR operation of the inverse of the first control signal and the inverse of the second control signal.
 14. The system of claim 8, wherein the second gate signal being based on the second control signal alone includes the second gate signal being at least one of: the second control signal, an inversion of the second control signal, and a delay of the second control signal.
 15. A method comprising: programming a plurality of control signals including a first control signal and a second control signal that specify a target resistance and a target voltage in a circuit; generating a plurality of gate signals including a first gate signal, a second gate signal, and a third gate signal, the first gate signal is generated based on a first logic operation of the first control signal and the second control signal, the second gate signal being based on the second control signal alone, and the third gate signal being based on a second logic operation of the first control signal and the second control signal; sending the plurality of gate signals to at least one transistor configured to control a current flow in the circuit; and providing, as an output, a signal with the target voltage and target resistance, wherein the first logic operation represents an AND operation of the second control signal and an inverse of the first control signal, and the second logic operation represents an OR operation of the inverse of the first control signal and the inverse of the second control signal.
 16. A system comprising: at least one transistor in a circuit; and a signal generation circuit configured to generate a plurality of gate signals, the plurality of gate signals including a first gate signal, a second gate signal, and a third gate signal, based on a plurality of control signals that specify a target resistance and a target voltage in the circuit, the plurality of control signals including a first control signal and second control signal, wherein the signal generation unit is further configured to generate the first gate signal based on a first logic operation of the first control signal and the second control signal, the second gate signal being based on the second control signal alone, and the third gate signal being based on a second logic operation of the first control signal and second control signal; wherein the at least one transistor is configured to receive, respectively, one of the plurality of gate signals, the circuit providing, as an output, a signal with the target voltage and target resistance; wherein the first logic operation represents an AND operation of the second control signal and the inverse of the first control signal, and the second logic operation represents an OR operation of the inverse of the first control signal and the inverse of the second control signal. 